Effect of exercise on erythrocyte beta-adrenergic receptors and plasma concentrations of catecholamines and thyroid hormones in Thoroughbred horses.
The effects of exercise stress on erythrocyte beta-adrenergic receptor characteristics and plasma concentrations of adrenaline, noradrenaline and thyroid hormones were studied in Thoroughbred racehorses during rest and after exercise. Five minutes after a maximal speed race of 1200 +/- 200 m (mean +/- s.d.), both plasma adrenaline and noradrenaline concentrations increased with respect to basal values (from 2.48 +/- 0.15 to 3.83 +/- 0.27 and from 2.13 +/- 0.11 to 3.53 +/- 0.27 nmol/l respectively). The increment of adrenaline was greater in high performance (HP) as compared to low performance (LP) horses (76.9 vs. 43.5%), in accordance with the contribution of the adrenal medulla in the sympathoadrenal response to exercise. Triiodothyronine (T3), but not thyroxine (T4) levels increased 5 min after exercise (from 55.6 +/- 2.9 to 81 +/- 3.7 ng/dl and from 0.67 +/- 0.04 to 0.70 +/- 0.05 micrograms/dl respectively). No differences were observed in basal values of thyroid hormones or in the percentage of T3 increment, when comparing HP vs. LP horses. Erythrocyte membranes obtained 5 min after racing showed decreased concentrations of beta-adrenergic receptors (beta-AR) and dissociation constant as compared to basal values (50.1 +/- 7.0 vs. 95.7 +/- 12.0 fmol/mg protein and 0.97 +/- 0.24 vs. 2.04 +/- 0.3 nmol/l respectively). This temporal pattern suggest that the observed changes in beta-AR characteristics could be mediated by catecholamines, but not by thyroid hormones, in this model. This down regulation of beta-AR may act as a protecting mechanism preventing the erythrocytes from the decrease in membrane fluidity known to be provoked by adrenergic agonists. The accomplished study showed that, in the Thoroughbred horse, there is a homeostatic response to race stress, characterised by a sudden increase in plasma catecholamines and T3 and a parallel decrease in beta-AR concentration on the erythrocyte membrane. In this way the racing horse could rapidly adjust its metabolism to the exercise stress, but at the same time override one possible undesirable side-effect caused by these hormonal changes. Further studies will be required to establish performance-related differences occurring in endocrine changes.
González
O
O
Department of Physiology and Biophysics, Faculty of Medicine, University of Chile, Santiago, Chile.
González
E
E
Sánchez
C
C
Pinto
J
J
González
I
I
Enríquez
O
O
Martínez
R
R
Filgueira
G
G
White
A
A
D000818:Animals; D002395:Catecholamines / Q000097:blood*; D015536:Down-Regulation; D004837:Epinephrine / Q000097:blood; D004912:Erythrocytes / Q000737:chemistry* / Q000378:metabolism; D005260:Female; D006736:Horses / Q000097:blood* / Q000378:metabolism / Q000502:physiology*; D008297:Male; D009638:Norepinephrine / Q000097:blood; D010805:Physical Conditioning, Animal / Q000502:physiology*; D011943:Receptors, Adrenergic, beta / Q000032:analysis* / Q000378:metabolism / Q000502:physiology; D012146:Rest / Q000502:physiology; D013963:Thyroid Hormones / Q000097:blood*; D013974:Thyroxine / Q000097:blood; D013997:Time Factors; D014284:Triiodothyronine / Q000097:blood
D002395:Catecholamines; D011943:Receptors, Adrenergic, beta; D013963:Thyroid Hormones; D014284:Triiodothyronine; D013974:Thyroxine; D009638:Norepinephrine; D004837:Epinephrine